This invention relates to light responsive transducers for converting optical energy (e.g., light, whether visible radiation or not) to electrical energy (e.g., a photovoltage or photocurrent).
Optical-to-electrical energy transducers cover a broad spectrum of devices ranging from electron photomultipliers to semiconductor photodetectors. The latter class includes two basic types of p-n junction devices: (1) the photovoltaic device (or solar cell) which requires no external voltage source but rather itself is a voltage source when exposed to radiation of a suitable spectral distribution; and (2) the photodiode which is widely used under reverse bias conditions for high-speed detection of both coherent and incoherent optical signals. These semiconductor devices have been fabricated in either a homojunction or heterojunction configuration from a variety of materials such as Si, GaAs/AlGaAs, CdS/Cu.sub.2 S, SnO/Si, InSnO/InP, and many others. Both configurations have utilized p-i-n multilayered structures in which radiation is made incident either normal to the layers, as in solar cell applications, or parallel to the layers, as in edge-illuminated photodiodes.
The p-i-n photodiode is the most common depletion-layer photodetector because the depletion region thickness can be tailored to optimize the sensitivity range and frequency response. In operation, radiation to be detected is absorbed in the i-layer where it generates electron-hole pairs. These pairs, which are produced in the depletion region or within a diffusion length of it, are eventually separated by the electric field, leading to current flow in the external circuit as carriers drift across the depletion layer. If the external circuit is open, an open circuit voltage will develop between the contacts to the n- and p-layers. The magnitude of this open circuit voltage V.sub.oc (the maximum output voltage) for the case of a single, ideal junction is given by ##EQU1## where I.sub.S is the reverse saturation current, n is the diode ideality factor, I.sub.L is the photocurrent generated by the device, q is the electronic charge, k is the Boltzmann constant, and T is the absolute temperature. From the above equation, it is apparent that for a fixed I.sub.L, V.sub.oc cannot be easily increased to arbitrarily large values because I.sub.S is a fixed physical parameter. For a single GaAs junction, for example, V.sub.oc is usually of the order of 1.1-1.2 V. But, higher output voltages may be desirable in some applications where, for example, the photodiode is used to power other electronic equipment as well as to perform signal detection. To attain higher voltages, a series of photodiodes may be connected in series. If the photodiodes are discrete, the configuration has the disadvantage that beam splitters are required to direct a portion of the radiation to each photodiode. On the other hand, if the photodiodes are stacked, then they must be interleaved with tunnel junctions as described by M. Illegems et al in U.S. Pat. No. 4,127,862, issued on Nov. 28, 1978, and assigned to the assignee hereof.
As mentioned previously, p-i-n photodiodes may be either normal incidence or edge-illuminated devices. However, edge-illuminated devices are not usually preferred because of the low optical coupling efficiency into the thin i-layer. On the other hand, coupling into a normal-incidence device is typically achieved through the hole in an annular metal contact on either the p- or n-layer, or through the spaces between a comb-like metal contact. The latter type of contact is commonly used in solar cells to reduce absorption in the metal and to achieve large area cells. This type of contact on a heterojunction solar cell is depicted in FIG. 7.1, page 134, of a book by B. L. Sharma et al, Semiconductor Heterojunctions (Pergamon Press, 1974). One difficulty with this type of structure arises from the wide bandgap top layer often used to reduce surface recombination at the light-incident surface of the narrower bandgap underlying layer; that is, the radiation to be detected is filtered by the top layer and thus, in solar cell applications especially, reduces the efficiency with which photocarriers are generated.
Another drawback to the normal-incidence scheme arises from the trade-off between speed of response and efficiency of absorption. More specifically, fast response requires that the carrier transit time delay in the depletion layer should be short. This end is accomplished by having the depletion layer thin. However, thin depletion layers do not efficiently or completely absorb the incident radiation, thus resulting in a relatively small photocurrent. Furthermore, a thin depletion layer increases the device capacitance, which further increases the RC time constant.
In contrast, with the edge-illuminated scheme, the incident radiation that falls on the i-layer is totally absorbed since in this parallel direction, the i-layer is thick. In the vertical direction, the depleted i-layer can be made thin so that the transit time delay is short. However, as pointed out above, because the i-layer is thin, most of the incident light is not coupled into the layer.